The present invention relates to communications and radar systems and, more particularly, to a method and apparatus for transmitting communications signals that are resistant to electronic interference.
All communications systems, civilian and military, must be able to operate in the crowded electromagnetic environment that results from the transmission of other radiation, both from the communicator's own band as well as from outside the bands. The communication link must be designed to reject those unwanted radiations and to minimize the likelihood of its own transmissions causing trouble to other users of the spectrum. Military communicators, however, must also operate in a hostile environment where they may be subjected to deliberate interference designed to degrade their performance. The various methods for interfering electronically with communications signals are called electronic countermeasures (ECM). Active ECM is sometimes referred to as jamming. The methods employed to combat ECM are called electronic counter-countermeasures (ECCM).
For example, jamming includes carrier wave (CW) jamming, broadband jamming, medium and fast sweep jamming, impulse jamming and short pulse jamming. Corresponding ECCM procedures have been developed to defend against each of those techniques.
Communications transmitters, receivers and processors are commonly constructed to permit the received signal to be distinguished from the jamming signal. Such distinctions between received signals and jamming signals may be effected by well known ECCM techniques in the construction of the transmitter, the antenna, the receiver and/or the data processor. Techniques such as direct sequence pseudonoise (DSPN) spread spectrum, frequency hopping, pulse compresssion, side lobe cancellation and others are commonly used to distinguish the received signal from one or more types of ECM interference.
In the constantly evolving and competing fields of ECM and ECCM it can be assumed that where an adversary is willing to pay the resultant price, sufficient ECM can be brought to bear against any single communication link to significantly reduce its effectiveness. The goal of ECCM is, therefore, not to attempt to assure communication system effectiveness in any environment, but to raise the cost of ECM to the point where it is prohibitive. In the process, ECCM techniques are intended to enable reliable communications despite any degradation caused by most hostile ECM. A more detailed discussion concerning the operation of ECM and ECCM is provided by Robert C. Dixon in "Spread Spectrum Systems" J. Wiley and Sons
In relation to the broad class of ECCM systems, the present invention is directed to a technique for generating a novel type of communications signal to be transmitted, which facilitates detection and isolation of an ECM signal from the transmitted communications signal by the receiver. As further described and illustrated below, conventional direct sequence pseudonoise spread spectrum communications signals are characterized by a ragged power spectral density having a distinct main lobe and side lobes. That raggedness is attributable to factors including the intentional randomness of the pseudonoise spreading stream (often referred to as a key stream) creating the transmitted communications signal. The raggedness of the power spectral density of the communication signal facilitates ECM interference, i.e. it complicates any attempt by a receiver to discern the spectral characteristics of a jamming signal accompanying the received communications signal. That difficulty is particularly acute where the ECM signal is a narrow band jamming signal that falls within the frequency band of the communications signal. The present invention is directed to apparatus and a technique for operating on the pseudo-random data signal to create a communications signal characterized by a smooth power spectral density over the signal bandwidth, such that ECM signals are more readily identified and isolated from the communications signal. The invention may be used in conjunction with existing signal processing equipment and other contemporary ECCM techniques.